1. Field of the Invention
The present invention relates to a device and method for monitoring, noninvasively and in a nonperturbating way, dynamic variations in viscoelastic-related properties of a tissue. More particularly, the present invention relates to an ultrasonic device that monitors continuously, noninvasively and without perturbing the blood flow, small-amplitude instantaneous fluctuations in the velocity and absorption of ultrasonic waves in a living tissue, caused by the blood microcirculatory system and to a method for the use thereof.
2. Discussion of the Prior Art
The supply of oxygen and nutrients to the body's tissues is mediated by a complex system of small blood vessels, i.e. microvessels, having diameters ranging from 0.005 to 0.5 mm. This so-called microcirculatory system modulates the distribution of blood by utilizing a sophisticated system of microscopic muscles, which are subject to neural control. The muscles change the diameter of some of the blood vessels (arterioles) and connect/disconnect the blood supply to the capillaries, causing the volume of fluids and its red blood cell content (hematocrit) to fluctuate in time (Zweifach, B. W. (1961)--Functional Behaviour of the Microcirculation, Charles Thomas, Springfield).
The resulting fluctuations in the blood supply and the related variations in the diameter of some of the microvessels are called vasomotion. Vasomotion is subject to neural control and its dynamic characteristics are significantly altered in case of some cardiovascular diseases including diabetes, hypertension, arteriosclerosis and related disorders [Davies, E., S. Ben-Hador and J. Landau (1962) 2nd Europ. Conf. Microcirculation, Pavia, Bibl. anat. 4, 195-200, Karger, Basel/New York (1964)].
Accumulating evidence supports the idea that vasomotion propels the blood and thus maintains its fluidity (Schmid-Schonbein, H. (1981) Interaction of vasomotion and blood rheology in haemodynamics. In: Clinical Aspects of Blood Viscosity and Cell Deformability. Eds. Lowe, G. D. O., Barbenel, J. C., Springer Verleg, pp. 49-67). This is because blood viscosity, which is an important clinical parameter, increases strongly at small velocity gradients.
Drugs, anaesthetics and emotional stress induce vasodilation or vasoconstriction of the microvessels, which is favorable in some cases and unfavorable and often harmful in others, e.g. overloading the heart by decreasing the peripheral resistance in case of massive vasodilation and causing insufficiency of blood supply to tissues in cases of prolonged vasoconstriction.
In addition to the above "active" and relatively slow changes in the microcirculatory system, some microvessels respond "passively" to the pulse pressure by changing their volume. The response involves the viscoelastic properties of the microvessel wall, which undergo variations in case of some cardiovascular diseases, e.g. in arteriosclerosis, and are strongly nonlinear as functions of the phase of the vessel i.e. being dilated, constricted or relaxed (Wiederheilm, C.A. (1964), Viscoelastic properties of relaxed and constricted arteriolar walls. 3rd European Conference on Microcirculation, Jerusalem, Bibl. anat. 7, 346-352 Karger, Basel/New York 1965). The pattern displayed by these volume fluctuations reflect the capability of the microcirculatory system to follow the pressure pulse, as generated at the heart and propagated along the circulation pathway.
The following can thus be concluded: (a) A wide range of clinical situations are associated with the dynamic behavior of the microcirculatory system. (b) Microcirculation dynamics involves variations in the volume and the viscoelastic properties of some of the tissue components. (c) One can infer from (a) & (b) that monitoring, at real time, physical parameters, which are related to fluctuations in the viscoelastic properties of a tissue, i.e. the sum of the volume-weighted contributions of its components, will prove to be extremely valuable for clinical medicine and diagnosis.
What is therefore needed is a method for continuous, noninvasive and highly-sensitive monitoring of fluctuations in viscoelastic-related properties of a tissue.